Most local-area networks have file servers that manage common disk space, making it easier to share files and perform backups for user clients. Standard UNIX network services such as mail and file transfer also use the client-server paradigm. This chapter discusses several common client-server models for providing services over existing network infrastructure. The models are implemented with the Universal Internet Communication Interface (UICI), a simplified API for connection-oriented communication that is freely available from the book web site. The UICI interface is then implemented in terms of stream sockets and TCP.